1. Technical Field
The present invention relates to optical lithography systems. More particularly, this invention relates to a method and system for inspecting a mask or printed wafer for defects.
2. Background Art
Since the beginning of the computer era, manufacturers in the field of integrated circuits (IC's) have sought to reduce the geometric size of the devices (e.g., transistors or polygates) present on integrated circuits. The benefits achieved in reducing device dimensions include higher performance in smaller packaging sizes. However, numerous limitations arise as manufacturers attempt to achieve smaller and smaller device sizes. One primary problem manufacturers face is that as smaller devices are sought, the precision required from the tools used to create IC devices increases beyond their capabilities. Given the prohibitive costs involved and potential time lost to wait for the next generation of tools, manufacturers are forced to find techniques and methods that will allow such tools to operate beyond their intended specifications.
One of the first steps in manufacturing integrated circuit chips involves the laying or designing of the circuits to be packaged on a chip. Most ICs arc designed using computer aided design (CAD) layout tools. CAD tools allow chip manufacturers to plan the layout of the circuits on a computer where they can be analyzed and stored. Once this step is finished, the designs need to be transferred onto the chip. Unfortunately, present day chip manufacturing tools often lack the capability to create increasingly complicated and compact IC chips.
The predominate method of creating devices on IC chips involves the use of masks (Note that plates and masking plates will also be used in the following discussion to refer to masks). In general, masks typically comprise a transparent substrate on which various "circuit" patterns, determined by a CAD system, are disposed. That "circuit" pattern is then transferred onto the surface of a silicon wafer. The transfer of the pattern from the mask to the silicon substrate is accomplished by passing visible, ultraviolet, or even x-ray radiation (e.g., light) through the mask and onto a silicon substrate containing a photoresist material. Because the mask contains a pattern made up of solid lines and clear space, only those areas made up of clear space will allow radiation to pass. This process results in the creation of devices on the silicon substrate. This methodology is referred to as photolithography.
A popular method of creating mask patterns involves the use of chrome and is often referred to as chrome on glass (COG). It is recognized however that the methods and systems described herein are equally applicable to all masks that involve light blocking materials and/or attenuated mask systems. In attenuated mask devices, such as attenuated phase shifters and alternating phase shifters, the chrome or other light blocking material is replaced with an attenuating material that allows a small amount of light (e.g., 6%) to pass through and get phase shifted. The materials may include silicon nitride, carbon, thin chrome with an oxide, thin chrome with the clear areas etched, etc.
Unfortunately, the efficacy of all lithography tools is limited by numerous factors, and is especially limited by the resolution of the lens, or reticles, used to direct the radiation through the mask. When a system is being used within its resolution limits, an aerial image of the circuit will be printed onto the chip as desired (i.e., "on size"). However, when the tool is being used aggressively, that is, past the design limits of the tool, certain images will print with a deviation from their desired size. This is referred to as operating in a nonlinear regime. Thus, under certain circumstances, it is not unusual to have polygates deviate from their desired size by as much as 50 nanometers (nm), which is unacceptably high.
There have been numerous attempts at solving this problem including those involving optical proximity corrections. Proximity correction techniques work by modifying the dimensions of the chrome lines on the mask to compensate for the error caused by nonlinear operation. Thus, under this technique, it may be necessary to put a chrome line with a width of 0.95 microns on the mask to print a line with a width of 1.0 microns. However, because a given mask may contain millions of lines of varying dimensions, difficulties arise in providing an efficient and accurate method for calculating line modifications.
To fabricate advanced mask devices, the use of several optical proximity correction techniques are required. These include FIG. 1 which discloses an anchor 10 which is used to prevent line shorting, FIGS. 2 and 3 disclose serifs 11 to better define corners, FIG. 4 discloses a jog 12 to better control line width, and FIGS. 5 and 6 disclose outriggers 13 to improve image size and square. These correction techniques significantly improve wafer lithographic performance, but due to their sub-resolution sizes create a mask challenge to write, inspect, and repair.
Inspection problems using optical proximity correction include Die-to-Data inspection because of the mismatch between the data and the processed image on the mask. (Die-to-Data inspection involves comparing glass representation of a desired image from a computer to images obtained from an inspection tool). The mismatch between the data and the processed image is made worse when optical proximity correction features are added to the mask design. This results in the inspection equipment finding too many false defects. Any attempt to relax the inspection criteria will allow other defects to be missed. Therefore, current Die-to-Data inspection techniques do not allow for accurate Die-to-Data inspection on masks due to process and tool anomalies.
Another inspection problem involves contact-like level masks. Contact-like level masks brings metal connections underneath to a surface above. A problem occurs where the contact shape and area are skewed and do not fall within the defect tolerances of the data, but are still functional.